Apparatus and method for managing a microprocessor providing for a secure execution mode

ABSTRACT

An apparatus providing for a secure execution environment including a microprocessor and a secure non-volatile memory. The microprocessor executes non-secure application programs and a secure application program. The non-secure application programs are accessed from a system memory via a system bus. The secure application program executes in a secure execution mode. The microprocessor has secure execution mode logic that monitors conditions corresponding to the microprocessor associated with tampering, and causes the microprocessor to transition to a degraded operating mode from the secure execution mode following detection of a first one or more of the conditions. The degraded operating mode exclusively provides for execution of BIOS instructions. The secure non-volatile memory is coupled to the microprocessor via a private bus, stores the secure application program. Transactions over the private bus are isolated from the system bus and corresponding system bus resources within the microprocessor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/055,980 (Docket: CNTR.2322), filed on May 24, 2008, which isherein incorporated by reference for all intents and purposes.

This application is related to the following co-pending U.S. patentapplications, each of which has a common assignee and common inventors.

SERIAL FILING NUMBER DATE TITLE       Oct. 31, 2008 MICROPROCESSORHAVING SECURE NON- (CNTR.2322) VOLATILE STORAGE ACCESS       Oct. 31,2008 MICROPROCESSOR HAVING INTERNAL (CNTR.2436) SECURE CACHE       Oct.31, 2008 MICROPROCESSOR APPARATUS PROVIDING (CNTR.2437) FOR SECUREINTERRUPTS AND EXCEPTIONS       Oct. 31, 2008 MICROPROCESSOR APPARATUSFOR SECURE (CNTR.2438) ON-DIE REAL-TIME CLOCK       Oct. 31, 2008 ON-DIECRYPTOGRAPHIC APPARATUS IN A (CNTR.2439) SECURE MICROPROCESSOR      Oct. 31, 2008 APPARATUS AND METHOD FOR ISOLATING (CNTR.2444) A SECUREEXECUTION MODE IN A MICROPROCESSOR       Oct. 31, 2008 MICROPROCESSORPROVIDING ISOLATED (CNTR.2445) TIMERS AND COUNTERS FOR EXECUTION OFSECURE CODE       Oct. 31, 2008 INITIALIZATION OF A MICROPROCESSOR(CNTR.2446) PROVIDING FOR EXECUTION OF SECURE CODE       Oct. 31, 2008MICROPROCESSOR APPARATUS AND (CNTR.2447) METHOD FOR PERSISTENTENABLEMENT OF A SECURE EXECUTION MODE       Oct. 31, 2008 TERMINATION OFSECURE EXECUTION (CNTR.2466) MODE IN A MICROPROCESSOR PROVIDING FOREXECUTION OF SECURE CODE       Oct. 31, 2008 APPARATUS AND METHOD FORDISABLING (CNTR.2470) A MICROPROCESSOR THAT PROVIDES FOR A SECUREEXECUTION MODE       Oct. 31, 2008 DEGRADED MODE OPERATION OF A(CNTR.2471) MICROPROCESSOR THAT PROVIDES FOR A SECURE EXECUTION MODE      Oct. 31, 2008 APPARATUS AND METHOD FOR (CNTR.2478) PRECLUDINGEXECUTION OF CERTAIN INSTRUCTIONS IN A SECURE EXECUTION MODEMICROPROCESSOR

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to the field of microelectronics, andmore particularly to a microprocessor that provides a secure executionmode of operation that allows code to be executed in a highly secureenvironment within the microprocessor itself.

2. Description of the Related Art

The use of desktop, laptop, and handheld computing and communicationdevices as platforms for digital communication of sensitive orproprietary data and digital rights controlled content continues todrive security innovations in the computing industry. For example, thereare numerous applications which are disturbed at no cost over theInternet for the purposes of downloading and managing digital audio andvideo files. Through these applications, a user is allowed limitedrights to songs, television shows, and movies. And a great deal ofattention is paid to protecting those rights through the use of securityfeatures built into the applications which often rely on securitymechanisms provided by their host platforms.

In addition to the protection of digital content rights, another factorwhich continues to drive improvements in computer system security is userestrictions which may be applied to the host platforms themselves. Itis a well known fact that the cell phone industry has provided forso-called “pay-as-you-go” use of particular communication devices. Withsuch a plan, a user pays no monthly fee, but instead pre-pays for acertain number of cellular minutes. When the minutes are exhausted, theuser is denied access to the cellular network for any calls other thanemergency calls.

As early as 2006, MICROSOFT® Corporation along with partnercorporations, began to provide “pay-as-you-go” personal computing whichis primarily directed toward emerging computer markets. Under thisscheme, a user pays for their computer as it is used, through thepurchase of prepaid cards. In addition, U.S. Patent ApplicationPublication 20060282899, which is assigned to MICROSOFT, describes asystem and method for delivery of an modular operating system, whichincludes a core function module, or basic kernel, providing fundamentaloperating system support and one or more add-on modules that allowcustomization of the operating system as desired. In this application,add-on modules may provide support or extended capability to thecomputer including hardware, applications, peripherals, and support. Adigital signature may be used to confirm the integrity of an add-onmodule prior to installation, and certification may be verified todetermine if installation of the add-on module is authorized. Bywithholding certification, a service provider may manage illegal orundesired modifications to a provided computer. In addition, digitalrights management may be used to enforce terms of use of the add-onmodule in keeping with licensing arrangements.

Not surprisingly, a veritable host of techniques have been developed aswell which provide for circumventing the security measures that are nowin place to protect and control access to rights controlled digitalmedia, communication devices, and computing platforms. In more recenttimes, “hacking” has become a bona fide field of study. In fact, thepresent inventors have noted the publication of numerous works which arededicated to tampering with or altogether defeating the securitymeasures that have been put into place to safeguard access to and/or useof protected assets. One such work is the book, Hacking the Xbox: AnIntroduction to Reverse Engineering, by Andrew Huang, San Francisco: NoStarch Press, 2003. The book focuses in particular on teaching hackingtechniques to overcome the security mechanisms of the XBOX® gamingplatform produced by MICROSOFT, and additionally provides significantteaching on the subjects of computer security and reverse engineering ingeneral, with a discussion of the vulnerabilities of so-called “secure”computing platforms.

Consequently, platform architects and designers continue to pursuetechniques and mechanisms that are more effective at protecting againstunauthorized platform access, regardless of whether the access is benign(e.g., probing, snooping), malicious (e.g., destructive orrights-defeating hacks), or somewhere in between (e.g., tampering). Manyof these mechanisms are directed at preventing an intruder fromphysically accessing a platform, such as housing the platform in asecure chassis (e.g., a locked metal enclosure) or encapsulatingvulnerable circuits in epoxy. But it is well known that these types oftechniques add both cost and complexity to a system. Alternativemechanisms utilize security features that are provided for in aparticular computer architecture itself.

Consider the two primary security features provided for by thewell-known x86 architecture: paged virtual memory and privilegedexecution. Under paged virtual memory, the underlying operating systemdefines a separate virtual address space along with access rights (e.g.,execute only, read only) for each application that is being executed,thus precluding another surreptitious application from executing withinthe defined space or modifying its data. But the data associated withvirtual address translation (i.e., page tables) can be easily snoopedand changed since the data is resident in system memory and is presentedexternal to the host microprocessor on its system bus.

With privileged execution, the architecture provides several levels ofexecution privileges, CPL0 through CPL3. Accordingly, certain systemresources and instructions may only be accessed by applications whichare executing at higher privilege levels. It is common to find operatingsystem components operating at the highest privilege level, CPL0, anduser applications relegated to the lowest privilege level, CPL3. But asone skilled in the art will appreciate, these architectural featureswere developed primarily to preclude system crashes due to softwarebugs, and are not very effective at preventing intentional or directedhacks.

As a result, various methods and apparatus have been developed which aremore closely concentrated toward the prevention of intentional intrusionor takeover of a platform. In U.S. Pat. No. 5,615,263, Takahashi teachesa secure mode within a dual mode processor (i.e., microprocessor). In ageneral/external mode, the dual mode processor executes instructionsprovided from an external source. The instructions are supplied to theprocessor via input/output to the processor. Upon receiving a specialsoftware or hardware interrupt, the dual mode processor enters asecure/internal mode. The interrupt specifies a secure function storedin a read-only memory within the dual mode processor. Upon receivingsuch an interrupt, input/output to the dual mode processor is disabled.The identified secure function is executed by the processor. Duringexecution of the secure function, any attempt to insert instructions notoriginating from the read-only memory are ignored. However, theprocessor may access data specifically identified by secure functionbeing executed. Upon completion of performance of the secure function,an exit routine is executed to enable input/output to the processor andresume execution of instructions provided via input/output from thesource external to the processor.

Takahashi teaches that the secure mode is to be used for encryption anddecryption and the dual mode processor relies upon normal instructionsand data to be provided from an external control channel processor via abus conforming to a standard bus architecture such as the IndustryStandard Architecture (ISA). The dual mode processor powers up innon-secure mode and the secure mode is initiated via a software or ahardware interrupt. In secure mode, a limited number of functions (i.e.,instructions) related to encryption and decryption can be executed.These functions are stored within a read-only memory (ROM) which isinternal to the dual mode processor. As such, the present inventors havenoted that Takashi's dual mode processor is inadequate in that it canonly perform the limited number of functions which are provided forwithin it's internal ROM. Thus, an application comprising generalpurpose instructions cannot be executed in secure mode.

In U.S. Pat. No. 7,013,484, Ellison et al. teach a chipset forestablishing a secure environment for an isolated execution mode by anisolated storage, which are accessible by at least one processor. The atleast one processor has a plurality of threads and operates in normalexecution mode or the isolated execution mode. The secure environment ofEllison et al. relies upon an external chipset (“isolated executioncircuit”) which provides the mechanism for a processor to operate inisolated execution mode. The external chipset thus configures a securememory area, it handles decoding and translation of isolatedinstructions, generation of isolated bus cycles, and generation ofinterrupts. While the external chipset indeed provides for proactivesteps to isolate memory areas, instruction execution, and the like, itis noted that the external chipset is coupled to the at least oneprocessor via a typical system bus, thus allowing for bus snooping andtampering with traffic on the bus itself during execution of any securethread.

In U.S. Pat. No. 7,130,951, Christie et al. teach a method forcontrolling a secure execution mode-capable processor including aplurality of interrupts to interrupt the secure execution mode-capableprocessor when it is operating in a non-secure execution mode. Themethod includes disabling the plurality of interrupts from interruptingthe secure execution mode-capable processor when it is operating in asecure execution mode. And while disabling interrupts is a desirablesecurity feature in a secure execution environment, the processoraccording to Christie et al. relies upon instructions and data to beprovided via an operating system over a system bus. Once theinstructions are provided, then interrupts are disabled. Like themechanism of Ellison et al., such an approach is clearly susceptible tobus snooping and tampering with any of the instructions which are passedto the processor over the bus.

In U.S. Pat. No. 6,983,374, Hashimoto et al. teach a tamper resistantmicroprocessor that saves context information for one program whoseexecution is to be interrupted, where the processor state is encryptedand stored to system memory. Hashimoto also teaches a technique forfetching encrypted instructions from system memory and apparatus fordecrypting and executing the decrypted instructions. In addition,Hashimoto teaches using a symmetric key to provide the encryptedinstructions in memory and then using an asymmetric key algorithm toencrypt the symmetric key, which is stored in memory. The symmetric keyis known to the program creator and is encrypted using a public key thatis read from the processor. The processor includes a unique private keythat corresponds to the public key, which cannot be accessed by theuser. Accordingly, upon execution of a branch instruction, programcontrol is transferred to a “start encrypted execution” instructionwhich passes a pointer to the encrypted symmetric key. The processorfetches the encrypted symmetric key and decrypts it using its internalprivate key. Subsequently, the encrypted program instructions arefetched from system memory, decrypted using the decrypted symmetric key,and executed by the processor. If an interrupt or exception occurs, thestate of the processor is symmetrically encrypted and saved to memory.Hashimoto teaches the use of common cache mechanisms, interrupt logic,and exception processing logic for both unencrypted and encrypted code.

The present inventors have noted that the microprocessor of Hashimoto islimiting in that the symmetric key corresponding to secure code is knownby the code's creator, and could be compromised, thus exposing allsystems having that code to security attacks. In addition, the presentinventors have noticed that the processor of Hashimoto isdisadvantageous in that decryption of secure code must be executed onthe fly as instructions are fetched, which is extremely time consuming,thus causing throughput of the microprocessor to slow to a crawl.Furthermore, it is noted that the secure code of Hashimoto utilizesexisting non-secure resources such as system memory, page tables,interrupt and exception mechanisms, all of which are subject tosnooping.

Accordingly, the present inventors have noted that it is clearlydesirable to provide a microprocessor which is capable of executing anapplication or application thread comprising general purposeinstructions (i.e., any of the instructions in the microprocessor'sinstruction set) within a secure execution environment.

It is additionally desirable that the secure execution environment beisolated from any of the known methods of snooping and tampering. Thus,it is required that instructions executed by a secure execution modemicroprocessor be isolated from hardware within the microprocessor thatprovides access such as cache snoops, system bus traffic, interrupts,and debug and trace features.

It is furthermore desirable when applications are loaded for secureexecution by the microprocessor, that a mechanism is provided toobfuscate the structure and content of the applications from any extantobservation means and that a mechanism be provided to authenticate thesource of the application and to confirm its veracity.

SUMMARY OF THE INVENTION

The present invention, among other applications, is directed to solvingthe above-noted problems and addresses other problems, disadvantages,and limitations of the prior art. The present invention provides asuperior technique over that which has heretofore been provided thatenables the execution of secure application programs in a generalpurpose microprocessor platform. In one embodiment, an apparatusproviding for a secure execution environment is presented. The apparatusincludes a microprocessor and a secure non-volatile memory. Themicroprocessor is configured to execute non-secure application programsand a secure application program, where the non-secure applicationprograms are accessed from a system memory via a system bus, and wherethe secure application program is executed in a secure execution mode.The microprocessor has secure execution mode logic that is configured tomonitor conditions corresponding to the microprocessor that areassociated with potential security exposure and tampering, and that isconfigured to cause the microprocessor to transition to a degradedoperating mode from the secure execution mode following detection of afirst one or more of the conditions, where the degraded operating modeexclusively provides for execution of BIOS instructions, the BIOSinstructions comprising instructions to allow for user input and displayof messages. The secure non-volatile memory is coupled to themicroprocessor via a private bus, and is configured to store the secureapplication program, where transactions over the private bus between themicroprocessor and the secure non-volatile memory are isolated from thesystem bus and corresponding system bus resources within themicroprocessor.

One aspect of the present invention contemplates a microprocessorapparatus, for executing secure code within a secure executionenvironment. The microprocessor apparatus includes a secure non-volatilememory and a microprocessor. The secure non-volatile memory isconfigured to store a secure application program. The microprocessor iscoupled to the secure non-volatile memory via a private bus, and isconfigured to execute non-secure application programs and the secureapplication program. The microprocessor has secure execution mode logicthat is configured to monitor conditions corresponding to themicroprocessor that are associated with potential security exposure andtampering, and that is configured to cause the microprocessor totransition to a degraded operating mode from the secure execution modefollowing detection of a first one or more of the conditions, where thedegraded operating mode exclusively provides for execution of BIOSinstructions, the BIOS instructions comprising instructions to allow foruser input and display of messages.

Another aspect of the present invention comprehends a method forexecuting secure code within a secure execution environment. The methodincludes providing a secure non-volatile memory for storage of thesecure code, where the secure code is accessed within the securenon-volatile memory via private transactions accomplished over a privatebus that is coupled between the secure non-volatile memory and amicroprocessor, and where the private bus is isolated from all systembus resources within the microprocessor and external to themicroprocessor, and where the private bus is observable and accessibleexclusively by secure execution logic within the microprocessor;monitoring conditions corresponding to the microprocessor that areassociated with potential security exposure and tampering; and firsttransitioning the microprocessor to a degraded operating mode from thesecure execution mode following detection of a first one or more of theconditions, where the degraded operating mode exclusively provides forexecution of BIOS instructions, the BIOS instructions comprisinginstructions to allow for user input and display of messages.

Regarding industrial applicability, the present invention is implementedwithin a microprocessor which may be used in a general purpose orspecial purpose computing device.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features, and advantages of the presentinvention will become better understood with regard to the followingdescription, and accompanying drawings where:

FIG. 1 is a block diagram illustrating a secure execution mode (SEM)microprocessor according to the present invention;

FIG. 2 is a state diagram depicting top-level operating mode transitionsin the microprocessor of FIG. 1;

FIG. 3 is a block diagram illustrating details of SEM logic within themicroprocessor according to the present invention;

FIG. 4 is a block diagram showing how secure code is stored, accessed,initialized, and executed within the microprocessor according to thepresent invention;

FIG. 5 is a block diagram illustrating details of SEM watchdog logicwithin the microprocessor of FIG. 1;

FIG. 6 is a state diagram depicting detailed operating mode transitionswithin the microprocessor according to the present invention;

FIG. 7 is a flow chart showing a top-level method for enabling secureexecution mode operation in a microprocessor according to the presentinvention;

FIG. 8 is a flow chart highlighting a top-level method for disablingsecure execution mode operation in a microprocessor according to thepresent invention;

FIG. 9 is a flow chart featuring a detailed method for initializingexecution of secure code in a microprocessor according to the presentinvention;

FIG. 10 is a flow chart featuring a method for performing a resetoperation in a microprocessor according to the present invention thathas been enabled for SEM operation;

FIG. 11 is a flow chart featuring a detailed method for terminatingsecure execution mode operation in a microprocessor according to thepresent invention; and

FIG. 12 is a block diagram showing details of a secure real time clockin the microprocessor according to the present invention.

DETAILED DESCRIPTION

The following description is presented to enable one of ordinary skillin the art to make and use the present invention as provided within thecontext of a particular application and its requirements. Variousmodifications to the preferred embodiment will, however, be apparent toone skilled in the art, and the general principles defined herein may beapplied to other embodiments. Therefore, the present invention is notintended to be limited to the particular embodiments shown and describedherein, but is to be accorded the widest scope consistent with theprinciples and novel features herein disclosed.

In view of the above background discussion on secure or isolatedexecution of application programs in a microprocessor and the associatedpresent day techniques that are employed to prevent snooping, intrusion,tampering, or hacking, a discussion of the present invention will now bepresented with reference to FIGS. 1-12.

Referring to FIG. 1, a block diagram is presented illustrating a secureexecution mode (SEM) microprocessor 101 according to the presentinvention. The block diagram depicts a system board 100 (or,“motherboard”) to which the SEM microprocessor 101 is mounted. Themicroprocessor 101 is coupled to one or more bus masters 103 and/or toone or more bus agents 104 via a system bus 102. In one embodiment, theSEM microprocessor 101 is an x86-compatible microprocessor 101 that iscoupled to one or more x86-compatible bus masters 103 and/or one or morex86-compatible bus agents 104 via an x86-compatible system bus 102.

In addition, the SEM microprocessor 101 is coupled to a battery VP thatis mounted to the system board 100 and which couples to themicroprocessor 101 via connections VP1 and VP2. In one embodiment, thevoltage of battery VP is 1.8 VDC.

A crystal X1 is also mounted to the system board 100 and is coupled tothe microprocessor 101 via connections C1 and C2. The microprocessor 101includes SEM logic 105. The SEM logic 105 according to the presentinvention is configured to provide for initialization, operation, andtermination of a secure execution mode within the microprocessor 101 aswill be described in further detail herein below. The SEM logic 105comprises logic, circuits, devices, or microcode (i.e., microinstructions or native instructions), or a combination of logic,circuits, devices, or microcode, or equivalent elements that areemployed to initialize a secure execution mode, to load secureapplications for execution, to execute those applications in a secureenvironment, to monitor a number of microprocessor and system attributesin order to detect and preclude tampering, to terminate the secureexecution mode under appropriate conditions, and to halt processingaltogether if tampering is detected. The elements employed to performthese functions, and other functions within the SEM logic 105 may beshared with other circuits, microcode, etc., that are employed toperform other functions within the microprocessor 101. According to thescope of the present application, microcode is a term employed to referto a plurality of micro instructions. A micro instruction (also referredto as a native instruction) is an instruction at the level that a unitexecutes. For example, micro instructions are directly executed by areduced instruction set computer (RISC) microprocessor. For a complexinstruction set computer (CISC) microprocessor such as an x86-compatiblemicroprocessor, x86 instructions are first translated into associatedmicro instructions, and the associated micro instructions are thendirectly executed by a unit or units within the CISC microprocessor.

A secure non-volatile memory 107 is mounted to the system board 100 aswell, and is coupled to the microprocessor 101 via a private serial busPVT BUS 106 and a presence detection bus PSNT. According to the presentinvention, a non-volatile memory 107 is a memory whose contents persistthrough removal and reapplication of power. That is, the contents of thenon-volatile memory 107 do not change when power to the system board isturned off or when it is turned on. In one embodiment, the securenon-volatile memory 107 comprises a flash read-only memory (ROM) that issized commensurate with the size of the secure applications to beexecuted in secure execution mode. One embodiment contemplates a 4 MBflash ROM for the secure non-volatile memory 107. The transactions overPVT BUS are completely isolated from the system bus, the bus masters103, and bus agents 104. In one embodiment, the flash ROM 107 can beprogrammed up to 100,000 times. In one embodiment, the private bus 106contemplates a serial bus that provides for transactions between thesecure non-volatile memory 107 and the microprocessor 101. Such a bus106 may conform to a standard interface protocol such as the serialperipheral interface (SPI) protocol.

In operation, the battery VP and crystal X1 provide for persistentoperation of a secure real time clock (RTC) (not shown) within the SEMlogic 105, which will be described in more detail below. One or moresecure application programs, comprising instructions from the hostarchitecture instruction set, retrieved from system memory (not shown)over then system bus 102 and are stored into the non-volatile memory107. In one embodiment, the one or more secure application programs areencrypted via an asymmetric cryptographic algorithm using a private keybelonging to an authorizing entity and are accessed from the systemmemory in their encrypted form. One embodiment contemplates encryptionof the one or more application programs via the RSA algorithm. Acorresponding public key is employed by the microprocessor 101 todecrypt and validate the one or more secure application programs afterthey are fetched from the system memory. Upon enablement of secureexecution mode and upon execution of a “start secure execution”instruction, the SEM logic 105 employs cryptographic resources withinthe microprocessor to encrypt the one or more secure applicationprograms according to a symmetric key algorithm using a processor uniquecryptographic key and transfers the encrypted one or more secureapplication programs to the non-volatile memory 107 over the private bus106. Thereafter, the SEM logic 105 employs the cryptographic and otherresources within the microprocessor 101 to access, validate, and decryptthe one or more secure application programs, which are then loaded intoa secure and isolated random access memory (RAM), or cache (not shown)within the microprocessor 101.

When the start secure execution instruction executes, the SEM logic 105disables all system resources which allow for observation and/ortampering including interrupts, debug exception logic, trace logic, andthe like. The one or more application programs stored in the isolatedinternal RAM are executed using dedicated secure execution resourceswithin the SEM logic 105. The one or more application programs may thenreturn the processor state from secure operation mode to normalexecution mode, or they may place the processor 101 in a degraded modeof execution with limited functionality if potential tampering isdetected. If tampering is confirmed, then the SEM logic 105 causes themicroprocessor 101 to shut down altogether.

The types of functions envisioned for the one or more secure applicationprograms, or “secure code,” include, but are not limited to, performingcritical security tasks such as verifying certificates, encrypting data,decrypting data; monitoring normal system software activities; verifyingthe integrity of normal system software; tracking resource usage; andcontrolling installation of new software.

One embodiment of the secure processing system according to the presentinvention contemplates a surface mounted microprocessor 101, a surfacemounted secure non-volatile memory 107, and a surface mounted crystalX1. These surface mounted components comprise ball-grid array componentsor like technology which are soldered onto the system board 101, thusprecluding their removal via inexpensive methods.

The microprocessor 101 according to the present invention also executesnon-secure application programs which are stored in system memory (notshown), the instructions of which are provided over the system bus 102.In the sense of the present application, the microprocessor 101 iscapable of performing as a centralized processing unit (CPU) without arequirement for a coprocessor. That is, the microprocessor 101 accordingto the present invention is capable of executing all of the instructionsin the host instruction set and is capable of executing entireapplication programs. In contrast to coprocessors and processors of likefunction, which only executed single instructions, program threads, orprogram snippets that are handed off from a main CPU, the microprocessor101 according to the present invention directly executes all of theinstructions in a corresponding application program, regardless ofwhether that application program is a secure application programprovided via the secure non-volatile memory 107 or a non-secureapplication program fetched over the system bus 102.

Turning now to FIG. 2, a state diagram 200 is presented depictingtop-level operating mode transitions in the microprocessor of FIG. 1. Ata high level, the microprocessor 101 according to the present inventionprovides for three primary operating modes 201-203 and a shutdown mode204. A non-secure mode 201 is the first state that the microprocessor101 defaults to when first powered up after fabrication. The non-securemode 201 is also called “born free” mode 201. The born free mode 201 isthe manufactured state of the microprocessor 101 and provides for normalexecution of non-secure application programs, which are accessed insystem memory via the system bus 102. In this state, none of theresources associated with secure execution of secure applicationprograms are observable (i.e., visible to the non-secure applications)or operational. These resources include the SEM logic 105, the securememory 107, and a number of other dedicated registers includingregisters containing symmetric and asymmetric cryptographic keys, secureinterrupts, secure memory (RAM), and other hardware which will bediscussed in further detail below. By providing the born free mode, thetype of manufacturing activities common to a non-secure microprocessormay be applied. In addition, since the born free mode 201 provides forthe execution of non-secure secure application programs, the same diedesign for the microprocessor 101 according to the present invention maybe applied to a non-secure microprocessor. In one embodiment, the pinoutof the non-secure microprocessor is different from the SEMmicroprocessor 101 and if a non-secure version of the microprocessor ismounted to a secure system board 100, then the SEM logic 105 within thenon-secure microprocessor will render itself inoperable upon applicationof power.

In one embodiment, execution of a SEMENABLE instruction causes the modeof the microprocessor 101 to transition to secure mode 202. In thesecure mode 202, both secure and non-secure applications can beexecuted, but the non-secure applications do not have access to thesecure resources. The secure mode 202 is also called the SEM-enabledmode 202. Under the control of a secure application, the state of themicroprocessor may be changed back to the born free mode 201, however,the number of transitions back to born free mode 201 is finite. In oneembodiment, the processor may transition back to born free mode 201 upto 64 times. In an alternative embodiment, a write to a particularmachine specific register by a verifiable authorized entity cause themode of the microprocessor 101 to transition to secure mode 202.

If certain defined conditions are detected by the SEM logic 105, thenthe processor 101 automatically transitions to degraded mode 203. Indegraded mode 203, BIOS instructions are allowed to execute in order toallow for user input and the display of messages, but the execution ofmore complicated software such as an operating system is not allowed.Degraded mode 203 is provided so that secure code operating in thesecure mode 202 the microprocessor 101 may be shut down relative touseful work, but BIOS instructions are still allowed to execute. In oneembodiment, BIOS may be executed by forcing an external interrupt andcommunicating status via a machine specific register. In anx86-compatible embodiment, the SMI interrupt is forced in this mode 203to cause BIOS to execute.

The defined conditions that cause the microprocessor 101 to transitionfrom secure mode 202 to degraded mode 203 may be the result of theexecution of secure code, or hardware detected conditions, or acombination of secure code results and hardware detected conditions. Thehardware detected conditions comprise monitored conditions associatedwith potential security exposure or tampering. In addition, secure codeexecuting in the degraded mode 203 may cause the state to transitionback to secure mode 202.

Certain defined conditions related to configuration and integrity checksmay cause the processor 101 to transition to a hard shutdown mode 204.This mode can only be exited by a processor reset. Under the control ofa secure application program in either secure mode 202 or degraded mode203, the microprocessor 101 may enter the hard shutdown mode 204.

If certain defined conditions are detected by the SEM logic 105, thenthe processor 101 automatically transitions to degraded mode 203. Indegraded mode 203, BIOS instructions are allowed to execute in order toallow for user input and the display of messages, but the execution ofmore complicated software such as an operating system is not allowed.Degraded mode 203 is provided so that secure code operating in thesecure mode 202 the microprocessor 101 may be shut down relative touseful work, but BIOS instructions are still allowed to execute. In oneembodiment, BIOS may be executed by forcing an external interrupt andcommunicating status via a machine specific register. In anx86-compatible embodiment, the SMI interrupt is forced in this mode 203to cause BIOS to execute.

The defined conditions that cause the microprocessor 101 to transitionfrom secure mode 202 to degraded mode 203 may be the result of theexecution of secure code, or hardware detected conditions, or acombination of secure code results and hardware detected conditions. Thehardware detected conditions comprise monitored conditions associatedwith potential security exposure or tampering. In addition, secure codeexecuting in the degraded mode 203 may cause the state to transitionback to secure mode 202.

Certain defined conditions related to configuration and integrity checksmay cause the processor 101 to transition to a hard shutdown mode 204.This mode can only be exited by a processor reset. Under the control ofa secure application program in either secure mode 202 or degraded mode203, the microprocessor 101 may enter the hard shutdown mode 204.

Now referring to FIG. 3, a block diagram is presented illustratingdetails of SEM logic 301 within the microprocessor 301 according to thepresent invention. The SEM logic 301 includes and authorized partypublic key register 301, a unique processor symmetric key register 312,SEM initialization logic 305, SEM watchdog logic 306, SEM interruptlogic 307, SEM exception logic 308, SEM timers 309, a SEM real timeclock (RTC) 310, a non-volatile enabled indication register 328, a bankof SEM machine specific registers 329, and a secure volatile memory 302.The SEM logic 301 is coupled to a number of other resources within themicroprocessor 300 to include a non-secure memory 325 via bus 326,address logic 323 via bus 324, a random number generator 319 via bus320, an AES/Hash/RSA unit 311 via bus 321, other processor executionunits 313 (e.g., integer unit, floating point unit, MMX/SSE units) viabus 327, normal mode exception logic 314, normal mode trace/debug logic315, normal mode interrupt logic 316, and normal power management logic322.

In one embodiment, a public key is provided by an authorizing party andmay be permanently programmed into the authorized public key register318 during fabrication of the microprocessor 300. In one embodiment, thepublic key is a 1024-bit RSA key and the authorized public key register301 comprises a 1024-bit fuse bank. Thus, the key can be programmedduring fabrication of the microprocessor 300, but not thereafter.Alternatively, the public key can be programmed into the securenon-volatile memory 107 by an “off-line” mass initialization used toprogram a number of secure non-volatile memories 107. The ability toenable and initialize the secure mode 202 is a very critical securityoperation and presents a possible exposure to a Trojan Horseinstallation into the secure non-volatile memory 107. Thus, the publickey is provided to control the secure mode initialization process in amanner that precludes observation and tampering.

The processor key register 312 is an aggregation of a plurality of fuseswhich are physically distributed over the microprocessor die. The fusesare programmed at fabrication time with a unique and randomly generatedset of states to form a processor unique key which can only be read bythe cryptography/hash/Montgomery Multiplication unit 311. There is noprogram interface that provides for reading the processor key from thekey register 312. In one embodiment, the key register comprises 128fuses programmed with a 128-bit Advanced Encryption Standard (AES) keywhich is employed to encrypt and decrypt the contents of the securenon-volatile memory 107. That is the secure code is encrypted using thiskey for storage in the secure non-volatile memory. Upon retrieval of thesecure code via the private bus 106, the key from the processor keyregister 312 is employed to decrypt the secure code for execution.Consequently, an observer of the state of the private bus 106 would notbe able to determine what is being transferred between the processor 300and the non-volatile memory 107.

In one embodiment, the processor key register 312 comprises 128 polyfuses that are randomly distributed among the many other fuses within afuse bank in the microprocessor 300. The fuse bank is configured below anumber of metal layers on the microprocessor die.

The SEM initialization logic 305 provides for initialization of thesecure mode 202 upon execution of the SEMENABLE instruction, or othercontemplated mechanism for entering into the mode 202. For claritypurposes, operation of the microprocessor 300 according to the presentinvention will be described in terms of execution of instructions toenable (e.g., SEMENABLE) and executed code from the secure mode 202,however, on skilled in the art will appreciate that there are othermethods which may be employed to enable the secure mode 202 and executesecure code therefrom such as a write to a hidden register, etc. Uponsuccessful execution of a SEMENABLE instruction, the SEM initializationlogic 305 records the state of the microprocessor 300 in thenon-volatile enabled indicator register 328. Upon a return from securemode 202 to the born free mode 201, the SEM initialization logic 305records the state of the microprocessor 300 in the non-volatile enabledindicator register 328. In one embodiment, the non-volatile enabledindicator register 328 comprises a plurality of fuses and the number oftimes that the processor state can transition from secure mode 202 toborn free mode 201 corresponds to the specific number of fuses withinthe plurality of fuses.

The SEM watchdog logic 306 is employed to monitor the veracity of thesecure code and data and to monitor environmental and physicalattributes of the system including temperature, voltage, bus frequency,presence of the battery VP, presence of the crystal X1, and presence ofthe secure non-volatile memory 107. The SEM watchdog logic 306 indicatestampering or suspected tampering conditions to the SEM logic 301, whichmay cause the processor to transition to degraded mode 203 or toshutdown mode 204.

The SEM interrupt logic 308 provides for a plurality of interrupts andassociated interrupt logic (e.g., interrupt descriptor table (IDT))which are visible and accessible only by applications which areexecuting in the secure mode 202. The mechanism for interrupting securecode execution is similar to that of executing normal mode execution.That is, upon assertion of a SEM interrupt, the secure code state issaved, and transfer to the secure interrupt handler occurs through theSEM IDT occurs. Execution of a return from interrupt instruction returnscontrol to the interrupt point in the secure code.

Likewise, the SEM exception logic 308 provides for a plurality ofexceptions and associated exception handling logic which are visible andaccessible only by applications which are executing in the secure mode202. All secure code program exceptions and interrupts employ thepredefined IDT that is resident within the SEM interrupt logic 307 tocontrol branching during interrupts and exceptions. The secureinterrupts are configured to allow for program control transferresulting from events external to the microprocessor 300 such askeyboard events, I/O port events, and the like. The secure exceptionsare configured to allow for program control transfer resulting fromevents internal to the microprocessor 300 such as undefined opcodes,machine check errors, and in one embodiment, certain writes by securecode to one or more secure machine specific registers in the MSR bank329. The secure IDT comprises a plurality of secure registers into whichare loaded a plurality of pointers that point to secure interrupt andsecure exception handlers within the secure code.

The SEM timers 309 are a plurality of timers which are visible andaccessible only by applications which are executing in the secure mode202. The SEM timers 309 include interrupts which are accessible bysecure code operating in secure mode 202. The SEM real time clock (RTC)310 is a persistent RTC that is visible and accessible only byapplications which are executing in the secure mode 202. The value ofthe SEM RTC 310 cannot be changed by any entity other that secure codeoperating in secure mode 202. The SEM machine specific register (MSR)bank 329 comprises a plurality of MSRs which are visible and accessibleonly by applications which are executing in the secure mode 202. TheMSRs 329 are employed to enable load/store access to the securenon-volatile memory 107, the SEM RTC 310, and the SEM timers 309.

The non-secure memory 325 is employed as both an instruction and datacache for non-secure applications that are executing. These programs andother system bus resources within the microprocessor 300 can observe andaccess the non-secure memory 325. The secure volatile memory 302 isemployed as both an instruction and data cache for applications whichare executing in the secure mode 202. An area of the secure memory 302is provided to store processor state 303 upon entry into secure mode202. And areas of the memory 302 are provided for storage of secure code304 and data 317 corresponding to the secure code. The secure volatilememory 302 is cleared upon processor reset and it is entirely isolatedfrom the system bus and therefore cannot be snooped, loaded, debugged,or otherwise accessed by non-secure system resources. Secure code canaccess the secure volatile memory 302 to load/store data 317 by usingnormal processor load and store instructions that reference normalsegment registers within the address logic 323 which are initializedupon entry into secure execution to reference the secure volatile memory302 instead of normal system memory (not shown). The normal systemmemory can also be accessed by secure code executing in secure modeusing normal load and store instructions via the address logic 323.However, upon execution of the secure code, the SEM logic 301 directsthe address logic 323 via bus 324 to cease address translation. That is,the addresses provided over bus 324 by the secure code must be physicaladdresses because virtual-to-physical address translation is disabledfor both instructions and data. By doing so, the SEM logic precludespage faults, thereby eliminating this source of tampering.

In one embodiment, the secure volatile memory 302 resides completelywithin the on-chip caches in the microprocessor 300, but the securememory cache lines have a special internal attribute that completelyisolates them from the processor bus. These cache lines are not coupledto external system memory, and therefore they cannot be loaded fromsystem memory or stored to system memory, nor can they besnooped—externally or internally—by any bus snoop resource.

In one embodiment, the secure volatile memory 302 comprises 4K 64-bitcache lines. A cache line within the secure volatile memory 302 isallocated by moving data to a cache line that has not been previouslyreferenced.

In another embodiment, the secure volatile memory 302 comprises a randomaccess memory that is separate from the on-chip caches within themicroprocessor 300.

Execution of a SEMENTER instruction provides for execution of securecode within the secure mode 202. In an x86-compatible embodiment, thesecure mode 202 provides for execution of secure code according to amodified 32-bit x86 real mode, but entry into x86 protected mode isprecluded when secure code is executing. Prior to execution of securecode, the SEM initialization logic 305 disables non-secure (i.e.,“normal”) interrupt logic 316 by asserting signal DISIL. The SEMinitialization logic 305 also disables normal exception logic 314 byasserting signal DISEL and normal debug/trace logic 315 by assertingsignal DISDL. In addition, normal power management logic 322 is disabledvia assertion of signal DISPML. Through these security measures, normalbus interrupts cannot occur, debug exceptions are precluded, bus tracecycles are prevented, debug ports are disabled. In addition, signalDISDL is employed to disable all remaining processor resources (e.g.,JTAG, probe mode, cache test) during execution of secure code. The powermanagement logic 322 would otherwise allow the microprocessor 300 toenter into reduced power states, such as P-state and C-state in anx86-compatible embodiment. Hence, signal DISPML is employed to preventpower state transitions during secure code execution.

Via buses 320, 321, and 327, the secure code can access all of theexecution units 313 and special purpose units 319, 311 within the secureprocessor 300 to execute all of the instructions in the microprocessor'sinstruction set which include hardware generation of true random numbersand hardware implemented functions accessible by programmed macroinstructions to perform RSA encryption, decryption, and signatureverification; AES encryption and decryption; and SHA-1/SHA-256 hashgeneration. These hardware implemented functions are performed by theAES/HASH/RSA unit 311.

Now referring to FIG. 4, a block diagram 400 is presented showing howsecure code is stored, accessed, initialized, and executed within themicroprocessor according to the present invention. The block diagram 400depicts a SEM-capable microprocessor 401 that is coupled to BIOS memory410 and system memory 420 via a system bus 425. The microprocessor 401is also coupled to a secure non-volatile memory 430 according to thepresent invention via a private bus 431. The microprocessor 401 includessecure code interface logic 401 that is coupled to a random numbergenerator 412, a processor key register 403, an authorized public keyregister 404, an AES/HASH/RSA unit 405 (or, “cryptographic unit” 405),secure volatile memory 406, SEM watchdog logic 408, and SEMinitialization logic 409. The secure code interface logic 402 isadditionally coupled to a bus interface unit 403 and to a securenon-volatile interface unit 407.

The block diagram 400 also depicts secure code 411, 421 which is storedin both system memory 420 and BIOS memory 410. In one embodiment, thesecure code 411 stored within BIOS memory 410 is primarily employed toprovide for operation of the microprocessor 401 in the degraded mode203, and the secure code 421 stored in system memory 420 is employed foroperation of the microprocessor 401 in secure mode 202.

In operation, elements depicted in the block diagram 400 function in amanner substantially similar to like-named elements presented anddiscussed previously with reference to FIGS. 1-3. The intention of thisdiscussion with reference to FIG. 4 is to more specifically focus uponthose elements and techniques that are employed to store, access,initialize, and execute secure code within a secure environmentaccording to the present invention.

Accordingly, the environment for execution of secure code is isolatedfrom the environment for execution of non-secure code. As statedpreviously, the born free mode 201 allows only for execution ofnon-secure code. The secure mode 202 allows for execution of bothnon-secure code and secure code. Prior to the execution of secure code421, the state of the microprocessor 401 is saved. The state is restoredupon transition back to the execution of non-secure code. This state issaved in an area within the secure volatile memory 406, and the statenever appears on the processor bus 425. In addition, the secure code411, 421 is executed from the secure volatile memory 406. In addition toisolating the secure volatile memory 406 from hardware and softwareassociated with the processor bus 425, all other “side channels” such asdebug exceptions and execution trace features, are disabled, as notedabove in the discussion with respect to FIGS. 1-3. The secure code 411,421 is provided with exclusive access to the SEM interrupt logic 307,the SEM exception logic 308, the SEM RTC 310, the SEM timers 310, andother processor resources that are available only to secure applications411, 421.

In addition, the microprocessor 401 provides the SEM watchdog logic 408that includes asynchronous monitors and watchdog mechanisms whichexecute independently of the execution of the secure code 411, 421 ornon-secure code. The SEM watchdog logic 408 monitors the processor'senvironment (e.g., voltage, temperature, bus behavior) as well asverifying the integrity of the secure code 411, 421, as will bedescribed in more detail below. When a security exposure is detected,the SEM watchdog logic 306 can transfer control to a secure-code errorhandler within the secure code 411, 421 via bus CHK or, in the casewhere a severe security exposure is detected, the SEM watchdog logic 306will cause the microprocessor 401 to enter the degraded mode 203 via busCHK.

In one embodiment, the secure code interface logic 402 monitors thespecific instructions that are resident within the secure code 411, 421and provides these instructions to the SEM watchdog logic 408 via busINS in order to support restricted instruction set architecture (ISA)operation of the microprocessor 401. According to this embodiment, themicroprocessor 401 according to the present invention is only allowed toexecute certain instructions within the host ISA when it is operating inthe secure execution mode. That is, restricted ISA operation precludesthe execution of instructions that may present a security exposure forwhich the authorizing party wishes to preclude. For example, in an x86embodiment, it may desirable to preclude the execution of certaininstructions, or classes of instructions, which require the generationand execution of over, say, 100 microinstructions. On another hand, whenthe microprocessor 401 is operating in the secure execution mode, anauthorizing party may wish to preclude the execution of all instructionssuch as task switches, call gates, and the like. By providing each ofthe instructions in the secure code 411, 421 to the SEM watchdog logic408, the microprocessor 401 according to the present invention enablesrestricted ISA operation. In one embodiment, the instructions in therestricted ISA set (i.e., the instructions which are allowed forexecution in the secure execution mode) are indicated by the value of aninstruction array (not shown) within the SEM watchdog logic 408, as willbe described in more detail below. When a precluded instruction isencountered, the SEM watchdog logic 408 causes the microprocessor 401 toenter the degraded mode 203.

In one embodiment, the secure code interface logic 402 provides theinstructions in the secure code 411, 421 to the SEM watchdog logicconcurrent with loading the secure code 411, 421 into the securevolatile memory 406 for subsequent execution.

The ability to enable and initialize the secure mode 202 is a verycritical security operation and would otherwise present a possibleexposure to a Trojan Horse installation into the area of memory 410, 420containing the secure code 411, 421. Advantageously, the microprocessor401 according to the present invention precludes such an exposure bycontrolling the secure mode initialization process via use of anasymmetric cryptographic algorithm and a corresponding set of asymmetriccryptographic keys. In one embodiment, the asymmetric key algorithm isthe RSA algorithm and the corresponding keys are 1024-bit RSA public andprivate keys generated by the authorizing party. The authorizing party,or authorizing entity, in one embodiment, provides the secured code 411,421 for execution. As noted above in the discussion with reference toFIG. 3, one of the two keys is stored during fabrication of themicroprocessor 401 within the authorized public key register 318 and isemployed to decrypt data according to the asymmetric key algorithm whichwas encrypted using the other asymmetric key (i.e., the private key) ofthe authorizing party.

Accordingly, in one embodiment, the operating system executes theSEMENABLE instruction (or similar mechanism). This instruction passes aSEM enable parameter which is encrypted via the private key of theauthorizing entity. The secure code interface logic 402 then accessesthe public key via the authorized public key register 404 and employsthe AES/HASH/RSA unit 405 to decrypt the SEM enable parameter. Uponverification of the decrypted SEM enable parameter, the SEMinitialization logic 409 initializes the secure mode 202. Otherwise theSEM initialization logic 409 directs the microprocessor 401 to returnsfrom the SEMENABLE instruction and the microprocessor 401 remains innon-secure mode 201. In one embodiment, a return code is provided thatindicates whether or not the authorization to enter secure mode 202 wasaccepted along with a corresponding error status (if any).

In an alternative embodiment, the authorizing party programs theauthorized public key into an authorized public key area 432 of thesecure non-volatile memory 430, as opposed to being directly programmedinto the authorized public key register 404 during fabrication of themicroprocessor 401. Accordingly, upon power up of the microprocessor401, the public key is detected and retrieved from this area 432 by thesecure non-volatile memory interface unit 407. The key is then burnedinto the authorized public key register 404 by the secure code interfacelogic 402 along with an parameter that indicates they key has beenprogrammed. This alternative embodiment allows for a more flexibledistribution of the public key at the level of fabrication of the securenon-volatile memory 430.

The SEMENABLE instruction (or alternative enable mechanism) also passesa pointer to the location of the secure code 411, 421 in BIOS memory 410or system memory 420 along with any initial secure data. The pointer anddata are formatted according to a predefined structure and are encryptedaccording to the asymmetric key algorithm as well. The encrypted pointerand data are decrypted and formatting is verified. Unsuccessfulverification results in a return with error code.

If the pointer and data are confirmed and verified for structure, thenthe secure code interface logic 402 directs the bus interface unit 403to retrieve the secure code 411, 420 from BIOS 410 and/or system memory420. The secure code 411, 421 has also been encrypted according to theasymmetric key algorithm using the authorizing party private key andmust comport with the predefined structure as well. The secure codeinterface logic 402 employs the authorized public key 404 and theAES/HASH/RSA unit 405 to decrypt the encrypted secure code 411, 421.Following verification of correct format, the secure code interface unit402 employs the AES/HASH/RSA unit 405 to encrypt the secure code anddata according to a symmetric cryptographic algorithm using the contentsof the processor key register 403 as the symmetric key. As alluded toabove, the contents of the processor key register 403 is a 128-bitrandomly generated key that is unique to the microprocessor 401 and thesymmetric cryptographic algorithm comprises the Advanced EncryptionAlgorithm (AES) using 128-bit blocks and electronic code book (ECB)mode. The symmetrically encrypted secure code is then written via thesecure non-volatile memory interface unit 407 to the secure non-volatilememory 430. The interface unit 402 additionally employs the AES/HASH/RSAunit 405 and the processor key 403 to generate a plurality of hashes ofselected portions of the secure code, and encrypts and writes theseplurality of hashes to the secure non-volatile memory 430 as well. Inone embodiment, the hashes are generated according to the SHA-1algorithm, although other embodiments are contemplated.

Furthermore, the SEM initialization logic 409 disables JTAG, probe mode,cache test, and other processor features that would allow forobservation of the secure code via the mechanisms discussed withreference to FIG. 3.

When the encrypted and hashed secure code has been written to the securenon-volatile memory 430, the microprocessor 401 sets the non-volatileenabled indicator (shown as 328 in FIG. 3) to indicate that theprocessor 401 is operating in secure mode 202 and the SEM initializationlogic 409 forces the microprocessor 401 to perform a RESET sequence.

Part of the RESET sequence causes the contents of the non-volatileenable register 328 to be read, and if the contents indicate that theprocessor 401 is in secure mode 202, then additional operations areperformed which are unique to operating in the secure mode 202.

Thus, the secure code 411, 421 is initially encrypted and loaded by theauthorizing party into memory 410, 420. When secure mode 202 is enabled,the microprocessor 401 retrieves and verifies the secure code accordingto an asymmetric key algorithm using a key provided by the authorizingparty. The code is then encrypted and hashed using a processor uniquekey according to a symmetric key algorithm, and the symmetricallyencrypted code is then written to the private non-volatile memory 430over the private bus 431.

As will be described in further detail below, when the secure code is tobe executed, it is retrieved by the secure non-volatile memory interfaceunit 407 from the secure non-volatile memory 430 and decrypted using theprocessor key 403, and it written to the secure volatile memory 406within the processor 401, which is totally isolated from all hardwareand/or software which would otherwise provide visibility into itscontents. The secure non-volatile memory 406 functions as both aninstruction and data cache for execution of the secure applications.

In one embodiment, the secure non-volatile memory interface unit 407comprises a plurality of machine specific registers that are madevisible exclusively to secure code which allow a secure application (orthe secure code interface logic 402) to perform loads and stores to thesecure non-volatile memory 430. That is, reads and writes to the securenon-volatile memory 430, according to this embodiment, are performed byperforming reads and writes to the hidden machine specific registers.

Advantageously, an authorizing party can bind secure operation of themicroprocessor 401 to a secure mode environment and the structure andfunction of the secure code are protected from any reverse engineeringand other tampering/hacking techniques because the transactions providedover both the system bus 425 and the private secure bus 431 areencrypted.

Referring now to FIG. 5, a block diagram is presented illustratingdetails of SEM watchdog logic 500 within the microprocessor of FIG. 1.The watchdog logic 500 includes a physical environment monitor 501 thatis coupled to the secure non-volatile memory via signal PSNT, to theexternal battery via signals VP1 and VP2, and to the external crystalvia signals C1 and C2. The physical monitor provides an output via busNOBOOT.

The watchdog logic 500 also includes a bus clock monitor 502 having afrequency reference unit 503 therein. The bus clock monitor 502 iscoupled to the bus clock provided to the microprocessor via signal BUSCLK and the output of the bus clock monitor is coupled to bus TAMPER.

The watchdog logic 500 also includes a processor voltage monitor 504that is coupled to the power and bus termination voltages provided tothe microprocessor from the system board via signals VDD and BUSTERM.The watchdog logic 500 additionally includes a temperature monitor 505that is coupled to processor temperature sensing logic (not shown) viasignal TEMP. The watchdog logic 500 furthermore includes a data monitor506 that is coupled to secure code interface logic 402 via bus CHK. Theoutputs of the bus clock monitor 502, processor voltage monitor 504,temperature monitor 505, and data monitor 506 are coupled to bus TAMPER.

The watchdog logic 500 moreover includes a secure time stamp counter 507that is coupled to a normal time stamp counter 508, signal CORE CLK anda ratio machine specific register (MSR) 509. The output of the securetime stamp counter 507 is coupled to bus TAMPER.

The watchdog logic 500 additionally includes an instruction monitor 511that is coupled to an instruction array 512 and to bus INS. As notedabove in the discussion with reference to FIG. 4, instructions within asecure application program are provided to the watchdog logic 500 whenthe microprocessor is executing in the secure execution mode to supportrestricted execution of instructions within the host ISA. The output ofthe instruction monitor is coupled to bus TAMPER.

Finally, the watchdog logic 500 has a type monitor 510 that is coupledto bus PINCHK and which generates an output over bus DESTRUCT.

Buses NOBOOT, TAMPER, and DESTRUCT are coupled to a watchdog manager511. In one embodiment, the watchdog manager 511 generates outputs onsignals CLASS1, CLASS2, CLASS3, and DISABLE.

In operation the watchdog logic 500 is configured to perform hardwareand software checks which monitor physical and temporal attributes ofthe microprocessor according to the present invention to detect,identify, and classify operating events that may indicate an insecureoperating environment for the secure code such as changing or removingthe external battery, the external crystal, or the secure non-volatilememory; replacing the secure microprocessor according to the presentinvention with an insecure microprocessor according to the presentinvention; modifying the bus clock frequency; tampering with theprocessor power supply (VDD); modifying the encrypted secure code eitherin the system memory, BIOS memory, or the secure non-volatile memory;and causing excessive calls to the secure code itself.

Accordingly, when operating in the secure mode, the physical monitor 501determines if the secure non-volatile memory is removed by monitoring tostate of signal PSNT. A deassertion of PSNT indicates removal.Similarly, signals VP1 and VP2 are monitored to determine if theexternal battery is either changed in voltage or removed. In oneembodiment, the value of VP1 is proportional to the battery voltage.Likewise, the states of signals C1 and C2 indicate the presence orabsence of the external crystal. If the physical monitor detects any ofthe above noted changes, then the specific change is output to busNOBOOT.

Also, when operating in the secure mode 202, the bus clock monitor 501evaluates the frequency of signal BUS CLK to determine both short termand long term integrity of the system bus clock which is provided to themicroprocessor via the system board. The bus clock is routed to the busclock monitor 502 via signal BUS CLK and the monitor 502 checks forshort term bus clock deviations using an internal phase locked loop (notshown) which is synchronized to the bus clock and which is employed togenerate an internal clock for the core microprocessor circuitry. Themonitor 502 determines if the bus clock is held flat for an undue numberof cycles, or if the clock-to-clock variation is more than acceptable.In one embodiment, a variation greater than six percent is deemedunacceptable. In addition the monitor 502 employs the frequencyreference unit 503 as a medium speed oscillator circuit that istemperature- and voltage-independent. The monitor 502 compares aderivative of the bus clock to the output of the frequency referenceunit 503 to determine if the frequency of the bus clock has experienceda gradual variation. If any of the above noted events occur, then theevent is reports via bus TAMPER to the SEM logic, which will eithercause the microprocessor to enter the degraded mode 203 or to entershutdown 204.

The processor voltage monitor 504 evaluates monitor the processor powerand bus termination voltages supplied to the processor and which areprovided via signals VDD and BUSTERM. Low and high limits for thesevoltages are programmed via machine specific registers (not shown).Excursions from these programmed limits are reported via bus TAMPER.

The temperature monitor 505 includes an accurate thermal monitormechanism that (in addition to normal thermal monitoring functions)constantly monitors die temperature against preset high and lowtemperature limits. The high and low temperature limits are storedwithin machine specific registers within the temperature monitor 505which can be written by secure code. Excursions from the preset high andlow limits are reported via bus TAMPER.

The data monitor 506 is configured to detect and report cryptographicand configuration errors associated with secure code and secure data.The errors are reported via bus TAMPER. Exemplary errors that arereported are errors associated with execution of the SEMENABLE andSEMENTER instructions, decryption errors detected when fetching thesecure code from memory, and hash and format errors in the secure code.

The secure time stamp counter 507 is configured to count the number ofcycles of signal CORE CLK when secure code is executing. The secure codecan perform a machine specific register write to the ratio MSR 509 thatestablishes a maximum ratio between the value of the normal time stampcounter 508, which counts the number of cycles of signal CORE CLK duringthe execution of both non-secure code and secure code. If the ratio isexceeded, thereby indicating that the secure code has been called morethat a prescribed number of times, then the secure time stamp counter507 reports this event via bus TAMPER.

The instruction monitor 511 is configured to validate the instructionswithin the secure application program against a subset of theinstructions within the host ISA, and to indicate when an instruction inthat secure application program that is not within the subset has beenprogrammed for execution. The subset of instructions which are allowedfor execution within the secure execution mode is indicated by the valueof the instruction array 512. In one embodiment, the subset includes oneor more specific instructions within the host ISA, as identified byopcode. In another embodiment, the subset includes one or moreinstruction categories, as identified by a microcode complexity value.In a third embodiment, the subset includes one or more tag codes, eachof which are associated with one or more instruction opcodes.

The subset of instructions for execution in the secure execution modeare identified by the value of the instruction array 512. In oneembodiment, the instruction array 512 comprises a machine specificregister which is initially written by the secure application program.In another embodiment, the instruction array 512 comprises a pluralityof fuses which are blown during fabrication.

During initialization of the secure execution mode, as the secure codeis being transferred from the secure non-volatile memory to the securevolatile memory for execution, a value corresponding to each of theinstructions within the secure code is provided by the secure codeinterface logic via bus INS to the instruction monitor 511. In oneembodiment, the value of INS directly indicates the opcode of each ofthe instructions. In another embodiment, the value indicates a class ofinstructions (e.g., simple, complex, etc.). In yet another embodiment,the value is a tag that corresponds to one or more instructions withinthe ISA.

In another embodiment, the value of each of the instructions within thesecure code is provided by the secure code interface logic on bus INSwhen the secure non-volatile memory is being programmed prior toexecution of the secure code.

The instruction monitor 511 compares the value of INS to the value ofthe instruction array 512 to determine if the particular instruction isallowed for execution. If not, then the instruction monitor 511 assertssignal TAMPER.

The type monitor 508 is provided to detect installation of a non-secureversion of the microprocessor according to the present invention into asystem board that is configured for a secure microprocessor according tothe present invention. In one embodiment, the non-secure microprocessorand the secure microprocessor have different pinout configurations. Thestates of particular pins that are different between the twomicroprocessor versions are provided as inputs to the type monitor 508via bus PINCHK. The type monitor evaluates the states of bus PINCHK andif it is determined that the non-secure version is installed, then thisevent is reported via bus DESTRUCT.

The watchdog manager 511 dynamically monitors the physical and operatingenvironment of the microprocessor by noting and evaluating the datacommunicated over buses NOBOOT, TAMPER, and DESTRUCT. Certain conditionsresult in assertion of signal CLASS1, such as a short deviation of thefrequency of BUS CLK, which is reported via TAMPER. The SEM logic, inresponse to assertion of CLASS1, logs the event in a secure event log inthe secure volatile memory and initiates an interrupt to the securecode. If the interrupt is not acknowledged, then the watchdog manager511 asserts signal CLASS3.

If more than one event is detected which would cause assertion of signalCLASS1, such as deviation of BUS CLK and deviation of VDD, then thewatchdog manager 511 asserts signal CLASS 2. The SEM logic, in response,attempts to erase the data area of the secure volatile memory andattempts to log the event in the non-volatile secure memory. Inaddition, hashes of the secure code in BIOS are checked. If the data issuccessfully erased and the event logged, and if the BIOS hashes areverified correctly, then the SEM logic initiates a transition to thedegraded mode 203, which provides for limited functionality, display oferrors, and limited user input. Failure of any of these actions resultsin assertion of signal CLASS3.

Assertion of signal CLASS3 indicates a critical security breach.Responsive to assertion of signal CLASS3, the SEM logic continues toattempt to erase the secure volatile memory and to log the event in thesecure non-volatile memory, and in addition, forces the processor intoshutdown mode 204, that is the processor becomes inoperative.

In one embodiment, the watchdog manager 511 determines if the typemonitor 510 has asserted signal DESTRUCT, thus indicating installationof an insecure version of the microprocessor according to the presentinvention. If DESTRUCT is asserted, and if the data on bus NOBOOTindicates that the external crystal and the secure non-volatile memoryare present, then signal DISABLE is asserted. Responsive to assertion ofsignal DISABLE, the SEM logic renders the non-secure processorinoperative.

The above conditions upon which the watchdog manager 511 asserts signalsCLASS1, CLASS2, CLASS3, and DISABLE are exemplary and are provided toteach secure environment management aspects of the present invention.One skilled in the art will however appreciate that classification ofsecurity events and appropriate responses are subject to the particularsecure environment that is required and thus, other methods ofclassification and response are comprehended by the present invention.

Now turning to FIG. 6, a state diagram 600 is presented depictingdetailed operating mode transitions within the microprocessor accordingto the present invention. The state diagram 600 includes the born freemode 601 (or, “non-secure” mode 601), the degraded mode 605, and thehard shutdown mode 606, as described for like named elements discussedwith reference to FIG. 2, with the difference being that the born freemode 601 is further detailed to indicate that only a limited number ofreturns to this mode can occur under program control. These limitednumber of returns are noted as BFM[1:N]. In addition, the secure mode202 described with reference to FIG. 2 is expanded in more detail todepict a plurality of SEM-enabled reset modes [1:N] 602, a SEM-enablednormal execution mode 603, and a SEM-enabled secure execution mode 604.That is, when secure mode 202 is enabled via execution of the SEMENABLEinstruction (or alternative enablement mechanism), the microprocessoraccording to the present invention may be in reset (i.e., ENABLEDRESET[1:N]), it may be executing non-secure applications (ENABLEDNORMAL), or it may be executing secure code (SECURE EXEC).

As noted above, the microprocessor according to the present invention isfabricated to enter the born free mode 601 upon initial power up. And asthe state diagram 600 indicates, the secure variant of themicroprocessor may be used and continue to be employed in the born freemode ad infinitum. However, execution of the SEMENABLE instruction oralternative mechanism for enabling secure mode (i.e., SEM ENABLE) causesthe microprocessor to enter a first one of the plurality of SEM-ENABLEDRESET modes 602, forcing a reset of the processor. In this mode 602,during reset sequence, the processor performs configuration andintegrity checks related to operating in a secure environment, as aredetailed above with reference to FIG. 5. Upon successful execution (i.e.PASS) of a RESET under the SEM-ENABLED RESET mode, the processortransitions to the ENABLED NORMAL mode 603 for execution of non-secureapplications. However, a if certain defined conditions are satisfied,such as are described above with reference to assertion of signalsCLASS3 and DISABLE by the watchdog manager, the processor willtransition to the degraded mode 605 (i.e., due to assertion of CLASS 3)or to a hard shutdown 606 (i.e., due to assertion of DISABLE). From hardshutdown 606, the processor may be RESET to cause it to return toENABLED RESET 602. From degraded mode 605, the processor allows forlimited interaction through the BIOS, allowing the user to establishparameters which enable to processor to enter ENABLED SECURE EXECUTIONmode 604 under program control.

From ENABLED RESET 602, a hardware call within the reset sequence willforce the processor directly into ENABLED SECURE EXECUTION mode 604,where secure code is executed. Additionally, a secure interrupt thatoccurs during execution of non-secure code in the ENABLED NORMAL mode603, or execution of a SEMENTER instruction or alternative mechanism fordirecting the processor to begin executing secure code will causetransition to the ENABLED SECURE EXECUTION mode 604. Collectively, theinstruction and alternative mechanisms for directing the processor tobegin executing secure code are referred to on the state diagram 600 asa “CALL.” Likewise, execution of a SEMEX1T instruction or alternativemechanism for directing the processor to terminate execution of securecode and to begin execution of non-secure code are referred to as a“RETURN” and such a RETURN causes the processor to transition to theENABLED NORMAL state 603. As noted above, the secure code may cause theprocessor to transition from ENABLED SECURE EXECUTION mode 604 toDEGRADED mode 605. And secure code within BIOS may allow for theprocessor to return to ENABLED SECURE EXECUTION mode 604 from DEGRADEDmode 605.

Finally, the secure code executing in ENABLED SECURE EXECUTION mode 604may cause a secure machine check exception by writing to a particularmachine specific register, which causes the processor to transition backto ENABLED NORMAL mode 603 for the execution of non-secure code.Additionally, if a secure device interrupt occurs in the ENABLED NORMALmode 603, then the state of the processor automatically changes toENABLED SECURE EXECUTION mode 604. The various steps which are performedin exemplary embodiments of the microprocessor according to the presentinvention to cause state transitions as described in the state diagramwill be discussed in further detail below with reference to FIGS. 7-11.

Referring now to FIG. 7, a flow chart 700 is presented showing atop-level method for enabling secure execution mode operation in amicroprocessor according to the present invention. Flow begins at block701 where the microprocessor is in the born free mode 601. Via executionof a SEMENABLE instruction or alternative mechanism (such as a write toa hidden machine specific register) for enabling the secure mode, aparameter is passed which has been encrypted according to an asymmetriccryptographic algorithm using one of a pair of asymmetric cryptographickeys, the other of which has been programmed into the authorized publickey register in the microprocessor. Flow then proceeds to block 702.

At block 702, utilizing the cryptographic unit within themicroprocessor, the parameter is decrypted to extract a valid command toenable secure mode along with a pointer to encrypted secure code inmemory. An additional pointer to secure code in BIOS may be provided aswell, along with any encrypted initialization data. Flow then proceedsto block 703.

At block 703, the encrypted secure code is retrieved from memory/BIOSvia the system bus and is decrypted. This secure code and data is nextencrypted according to a symmetric key algorithm using a processor keythat is unique to each microprocessor according to the presentinvention, and which is programmed into a processor key register atfabrication. The symmetrically encrypted code and data are subsequentlywritten over a private bus that is isolated from system bus resources toa secure non-volatile memory. Part of the process of writing to thesecure non-volatile memory includes performing a number of random writesto the memory prior to writing the symmetrically encrypted code anddata. Flow then proceeds to block 704.

At block 704, an non-volatile enabled register within the microprocessoris written to indicate that secure mode is enabled. In one embodiment,the register comprises a plurality of bits and one of the plurality ofbits is written to indicate that secure mode is enabled each time it isenabled. Another one of the bits is written to indicate a return to bornfree mode. Consequently, a 256-bit non-volatile register, according toone embodiment, would allow for 128 transitions from non-secure mode tosecure mode. Flow then proceeds to block 705.

At block 705, a processor reset is forced and the method completes.

FIG. 8 is a flow chart 800 highlighting a top-level method for disablingsecure execution mode operation in a microprocessor according to thepresent invention. That is, the flow chart 800 depicts how secure codeoperating in the secure mode may direct the microprocessor to return tothe born free mode. Flow begins at block 801 where secure code isexecuting in the secure mode. Flow then proceeds to block 802.

At block 802, the secure code executes a return to non-secure mode. Inone embodiment, a return to non-secure mode is initiated when the securecode performs a write to a SEM control machine specific register thatcauses a secure exception. Program control is then transferred to asecure exception handler within the secure code at an address indicatedby contents of the secure interrupt descriptor table discussedpreviously. In one embodiment, the secure exception handler performs awrite to a confirmation machine specific register to indicate acceptanceof the return. If the confirmation register is not correctly written,then the return is ignored and the processor remains in the secure mode.If the handshake is confirmed, then flow proceeds to decision block 803.

At decision block 803, contents of the non-volatile enable register areevaluated to determine if secure mode can or cannot be disabled. If not,then flow proceeds to block 806. If bits remain in the register to allowfor a return to non-secure mode, then flow proceeds to block 804.

At block 806, the secure mode is maintained and control returns to thesecure code. Flow then proceeds to block 807.

At block 804, the non-volatile enable register is updated to indicatethat the processor is operating in non-secure mode. Flow then proceedsto block 805.

At block 805, the state of the processor is returned to the born freemode. Flow then proceeds to block 806.

At block 806, the method completes.

FIG. 9 is a flow chart 900 featuring a detailed method for initializingexecution of secure code in a microprocessor according to the presentinvention. That is, the method of this flow chart 900 includes furtherdetails beyond that taught by the flow chart 700 of FIG. 7. Flow beginsat block 901 where a microprocessor according to the present inventionis executing a non-secure application in the born free mode. Flow thenproceeds to block 902.

At block 902, an operating system executing in the non-secure modeexecutes a SEMENABLE instruction or alternative mechanism, such as awrite to a machine specific register, that passes or otherwise transfersone or more parameters which have been asymmetrically encryptedaccording to a private key belonging to an authorizing party. The one ormore parameters include a pointer to asymmetrically encrypted securecode to be executed, which may be stored in system memory and/or BIOSmemory. Flow then proceeds to block 903.

At block 903, the microprocessor employs a corresponding public key todecrypt the passed one or more parameters. In one embodiment, the publickey is programmed during fabrication of the microprocessor into anon-volatile authorized public key register. In an alternativeembodiment, the public key is programmed into a location in a securenon-volatile memory according to the present invention and upon initialpower-up of the microprocessor, the public key is fetched from thesecure non-volatile memory, the key programmed into the non-volatileauthorized public key register, and then the location in the securenon-volatile memory is erased. Flow then proceeds to decision block 904.

At decision block 904, an evaluation is made to determine if thedecrypted parameter is valid. If so, then flow proceeds to block 905. Ifnot, then flow proceeds to block 907.

At block 905, since it was determined that the parameter is valid, thecontents of the secure non-volatile memory are erased by performing aplurality of random writes to all locations. Flow then proceeds todecision block 908.

At decision block 908, the encrypted secure code is fetched from thesystem memory and/or BIOS memory. The code is then decrypted accordingto the asymmetric key algorithm using the authorized public key. In oneembodiment, a cryptographic unit within execution logic in themicroprocessor is employed to decrypt the code. In one embodiment, thecryptographic unit is capable of performing AES cryptographicoperations, SHA-1 hash operations, and RSA cryptographic operations. Thedecrypted code is then decompressed and checked to determine if it isprovided according to a predefined format. If the decrypted code formatis correct, then flow proceeds to block 908. If the decrypted codeformat is not correct, then flow proceeds to block 907.

At block 907, since the decrypted parameter is invalid, program controlis returned to the operating system executing in non-secure mode. Flowthen proceeds to block 913.

At block 908, the decrypted code (and corresponding initialization data,if any) is encrypted according to a symmetric key algorithm using asymmetric key that is unique to the microprocessor and which wasprogrammed into a non-volatile processor key register at fabrication. Inone embodiment, the key is a 128-bit AES key and the microprocessoremploys its cryptographic unit to perform AES encryption of the code.Flow then proceeds to block 909.

At block 909, the microprocessor creates one or more hashes of one ormore sections of the encrypted code. In one embodiment, thecryptographic unit within the microprocessor is employed to create oneor more SHA-1 hashes of the encrypted code. Flow then proceeds to block910.

At block 910, the microprocessor writes the encrypted secure code (anddata, if any) and the one or more hashes to the secure non-volatilememory over a private serial bus that is isolated from system busresources. The code and data are encrypted, thus precluding detection ofthe content of the secure code. Flow then proceeds to block 911.

At block 911, an indicator within a non-volatile secure mode enabledregister is set to indicate that secure mode is enabled. Flow thenproceeds to block 912.

At block 912, a secure mode enabled reset sequence is performed withinthe microprocessor. The sequence includes hardware checks as aredescribed in the discussion referring to the watchdog logic of FIG. 5and initialization of the secure volatile memory to random values. Flowthen proceeds to block 913.

At block 913, the method completes.

Turning now to FIG. 10, a flow chart 1000 is presented featuring amethod for performing a secure mode enabled reset operation in amicroprocessor according to the present invention that has been enabledfor SEM operation. Flow begins at block 1001 where the secure modeenabled reset is forced by the microprocessor as it completesinitialization of the secure mode. Flow then proceeds to block 1002.

At block 1002, the processor performs a number of processor integritychecks to include detection and validation of the secure non-volatilememory, the external battery, and the external crystal. In addition, thepresence and frequency integrity of the bus clock is verified along withvalidation of proper voltages applied for bus termination and processorpower supply. The temperature of the processor is confirmed to be withinan acceptable range. Flow then proceeds to block 1003.

At block 1003, the processor reads and decrypts a secure signature froma location in the secure non-volatile memory. The decrypted signature isverified to confirm that the memory has not been compromised. Inaddition, specific portions of the secure non-volatile memory along withcorresponding hashes are read. Via the cryptography (i.e., AES/Hash/RSA)unit, verification hashes of the selected portions are generated andthen compared with the hashes that were read. Flow then proceeds toblock 1004.

At block 1004, correct operation of the secure real time clock isconfirmed as well. In one embodiment, the secure execution mode realtime clock evaluates the status of the external crystal to detect achange in frequency greater than five percent, thus indicatingcompromise of the crystal and any change in the external battery voltagegreater than five percent, thus indicating a potential security threat.If any of the above checks produce adverse results, the secure modeenabled reset sequence will cause either the event to be logged, or willforce the processor into the degraded mode, or will cause a hardshutdown, depending upon the severity and number of detected events.Flow then proceeds to block 1005.

At block 1005, the secure code is retrieved from system memory and/orBIOS memory. Flow then proceeds to block 1006.

At block 1006, the secure code is decrypted and decompressed and isverified for proper formatting. The secure code is then loaded into thesecure volatile memory within the microprocessor. Flow then proceeds toblock 1007.

At block 1007, secure resources within the microprocessor areinitialized. These secure resources are cannot be observed or accessedby non-secure code, and are available only to secure code executing inthe secure mode. These resources include secure timers, secureinterrupts and exceptions, to include a secure interrupt descriptortable, and any secure machine specific registers or other registers thatmust be initialized for execution of the secure code. Initializationalso includes disablement of non-secure interrupt, exception, debug, andtrace logic, along with disablement of any power management features ofthe microprocessor to include any elements which may cause a change incore voltage, core clock frequency, or which may enable or disable otherelements such as caches, branch prediction units, etc. Flow thenproceeds to block 1008.

At block 1008, the non-secure caches (i.e., L1 cache, L2 cache) withinthe microprocessor are initialized to random values. Flow then proceedsto block 1009.

At block 1009, a secure mode interrupt is generated and a secure codereset is called based upon the data resident in the secure modeinterrupt descriptor table which was initialized according to block1007. Flow then proceeds to block 1010.

At block 1010, the method completes.

Turning now to FIG. 11, a flow chart 1100 is presented featuring adetailed method for terminating secure execution mode operation in amicroprocessor according to the present invention. The method begins atblock 1101 where secure code is executing within the secure mode. Insummary, there are three ways according to the present invention for amicroprocessor to transition from non-secure mode to secure mode, and tobegin execution of secure code. A first way allows for program controlto transition to the execution of secure code. That is, a non-secureprogram in secure mode executes as SEMENTER instruction. In oneembodiment, the SEMENTER instruction is causes the state of themicroprocessor to be saved on a stack in the secure volatile memory andprogram control transferred to the secure code, very much like operationof the x86 SYSENTER instruction. A second way to cause the execution ofsecure code results from an interrupt or exception when performing anon-secure or secure reset sequence. A final way of causing theexecution of secure code results from an interrupt from any number ofsecure watchdog logic elements such as are discussed with reference toFIG. 5.

The secure code that is executed in secure mode, as noted above, residespermanently in the secure non-volatile memory, but has been loadedduring a secure mode enabled reset sequence into the secure volatilememory. That is, the secure code is never executed from non-securememory such as the system memory or non-secure processor caches.Consequently, execution control is transferred from secure mode back tonon-secure mode by two ways. The first way comprises executing anSRESUME instruction which cause a return from the SEMENTER instruction.In an x86 embodiment, the SRESEME instruction operates in a mannersimilar to an x86 RESUME instruction. That is, the program state whichwas previously stored in the secure volatile memory is restored andprogram control is transferred back to the operating system, ornon-secure code. A second way contemplates forcing a secure exceptionthat is accessible to the secure elements of the processor throughwriting a machine specific register which is accessible only to securecode. If it is confirmed that the microprocessor is to return to thenon-secure mode, then a non-secure machine check exception is generatedwhich is noted and processed by the operating system, thus affecting areturn to non-secure mode. The flow chart 1100 of FIG. 11 addressesforcing the secure exception to return to non-secure mode, although oneskilled in the art will appreciate that execution of the SRESUMEinstruction causes the microprocessor to perform similar steps as aredescribed below.

Accordingly, flow continues at block 1102, where the secure codeexecutes an instruction causing a write to a SEM MSR, that is, one of aplurality of machine specific registers which are accessible and visibleonly to secure code executing in the secure mode. Flow then proceeds toblock 1103.

At block 1103, writing the SEM MSR causes a secure exception which isprocessed by secure exception logic within the SEM logic. Flow thenproceeds to block 1104.

At block 1104, the secure exception logic (e.g., the secure interruptdescriptor table) causes program control to branch to a secure exceptionhandler within the secure code. Flow then proceeds to block 1105.

At block 1105, the secure exception handler executes a return back tothe secure code, passing an authorized exception code back to the securecode. Flow then proceeds to decision block 1106.

At decision block 1106, an evaluation is made to determine if theexception code returned by the secure exception handler is correct. Ifnot, then a security risk is assumed, and flow proceeds to block 1111.If the code is correct, then a handshake between the secure code and thesecure exception handler is confirmed and flow proceeds to block 1107.

At block 1107, the microprocessor purges the contents of the securenon-volatile memory by performing a plurality of random writes to alllocations therein. Flow then proceeds to block 1108.

At block 1108, the microprocessor erases each location in the securenon-volatile memory by writing zeros to each location therein. Flow thenproceeds to block 1109.

At block 1109, the non-volatile enabled register is written to indicatethat secure mode is disabled, subject to the number of times whichsecure mode may be disabled, as was previously discussed with referenceto the method of FIG. 8. Flow then proceeds to block 1110.

At block 1110, the secure exception logic generates a machine checkexception and in addition returns a status parameter. Thus, theoperating system in non-secure mode processes the machine checkexception, and a return to non-secure mode is accomplished. Flow thenproceeds to block 1111.

At block 1111, the method completes.

FIG. 12 is a block diagram showing details of a secure real time clock(RTC) 1200 that is within the SEM logic in the microprocessor accordingto the present invention. The secure RTC 1200 is exclusively visible toand accessible by secure code which is operating in the secure mode. Thesecure RTC is comprises an oscillator 1201 that is coupled to theexternal battery via signal VP and to an external crystal via signals C1and C2. The oscillator generates an output on signal VO which is coupledto a counter 1202. The counter generates and output on bus CNTO which isrouted to conversion logic 1203. Signals VP, C1, and C2 are also inputto the conversion logic 1203, along with a signal ENV, which carries avalue corresponding to the die temperature. The conversion logic 1203generates outputs that are provided over signals TIME, BATT, COMP, XTAL,and bidirectional bus TIME. The microprocessor provides inputs to thesecure RTC via bus TIME.

The oscillator 1201 and counter 1202 are dedicated, that is they shareno other circuitry with any other elements of the microprocessor exceptfor elements which are provided to allow the processor to read and writethe secure RTC via bus TIME. In addition, the secure RTC continues itscount as long as the external battery provides an acceptable voltage viasignal VP. In an alternative embodiment, voltage VP is generated by acapacitor on the system board provided in place of the battery which iskept charged as long as the system board is powered up.

In operation, the oscillator 1201 generates an oscillating voltageoutput that is proportional to the frequency of the external crystal andwhich is provided to the counter 1202. The counter 1202 includeselements that count the number of cycles provided via signal VO. Thecount is provided on signal CNTO. The conversion logic 1203 includescircuits that convert the value of CNTO to a time value, andadditionally includes registers (not shown) which can be read andwritten by the processor via bus TIME.

In addition, the conversion logic 1203 is configured to detect asignificant variation of voltage VP, indicating tampering, and such anevent is indicated by assertion of signal BATT which is employed tointerrupt the secure code that is executing. In one embodiment, avariation of greater than five percent causes the BATT interrupt to beasserted.

The conversion logic 1203 is also configured to detect a significantvariation of the crystal frequency via signals C1 and C1, thusindicating a potential tamper, and such an event is indicated byassertion of signal XTAL, which is employed to interrupt the secure codethat is executing. In one embodiment, a variation of greater than fivepercent causes the XTAL interrupt to be asserted.

The signal ENV is evaluated by the conversion logic 1203 to determinetemperature excursions that might make the count generated by thecounter 1202 inaccurate. If so, then signal TEMP is asserted, which isemployed to interrupt the secure code that is executing.

The conversion logic 1203 is also configured to evaluate if any of theabove noted conditions are significant enough to indicate that thesecure RTC has been compromised, such as removal and replacement of theexternal battery. If this is determined, then signal COMP is asserted aswell, thus interrupting execution of the secure code.

The present invention provides a number of advantages over present daytechniques for executing application programs within a secureenvironment. For example, the design according to the present inventionis processor based. That is, it is an object of the present invention tomodify the design of the processor responsible for executing the securecode because, in contrast to other techniques that focus on modifyingchipsets or other components, only the processor can provide realexecution security. Approaches using separate chips to monitor theprocessor have a number of inherent security flaws, as well assignificantly degraded performance for security related execution.

According to an x86 based embodiment of the present invention, thedevelopment of secure code is greatly simplified due to the ubiquitousnature of x86 programming skills. The x86 architecture is well known andthe addition of machine specific instructions and special instructionssuch as the SEMENABLE, SEMENTER, and SRESUME instructions provide only aminor learning challenge to any programmer well versed in thedevelopment of non-secure x86 applications.

Furthermore, the cost of addition secure execution capabilities to amicroprocessor is far less than what would be exhibited if additionalchips were added to a system design.

Moreover, since the secure execution environment is provided within theprocessor itself, it is inherently resistant against those physical orside-channel attacks which would otherwise require a significantinvestment in additional external circuits.

Very advantageously, the techniques disclosed herein provide for asecure microprocessor operating environment where no universal secret(such as a universal cryptographic key, or program structure) is storedwhich could be compromised. That is, each processor according to thepresent invention has only the secrets needed to authorize, control,etc. that particular processor or system. The secrets from oneprocessor/system will not break security on another processor/system.Further, knowing how to break security on one processor should not makeit any easier to break security on other processors. This is due to theunique processor key that is provided and resulting from transactionsover the secure non-volatile memory bus which are encrypted using thatkey.

The processor according to the present invention is additionallyadvantages over prior techniques in that protection against so calleddenial-of-service attacks is provided. For example, as discussed withreference to FIG. 5, secure watchdog elements are provided to detect andtake action on events such as continuous calls to the secure executionenvironment (from a rogue device driver, for example), continuous“removal” of the RTC battery, crystal, and etc.

Those skilled in the art should appreciate that they can readily use thedisclosed conception and specific embodiments as a basis for designingor modifying other structures for carrying out the same purposes of thepresent invention, and that various changes, substitutions andalterations can be made herein without departing from the scope of theinvention as defined by the appended claims.

1. An apparatus providing for a secure execution environment,comprising: a microprocessor, configured to execute non-secureapplication programs and a secure application program, wherein saidnon-secure application programs are accessed from a system memory via asystem bus, and wherein said secure application program is executed in asecure execution mode, said microprocessor comprising: secure executionmode logic, configured to monitor conditions corresponding to saidmicroprocessor that are associated with potential security exposure andtampering, and configured to cause said microprocessor to transition toa degraded operating mode from said secure execution mode followingdetection of a first one or more of said conditions, wherein saiddegraded operating mode exclusively provides for execution of BIOSinstructions, said BIOS instructions comprising instructions to allowfor user input and display of messages; and a secure non-volatilememory, coupled to said microprocessor via a private bus, configured tostore said secure application program, wherein transactions over saidprivate bus between said microprocessor and said secure non-volatilememory are isolated from said system bus and corresponding system busresources within said microprocessor.
 2. The apparatus as recited inclaim 1, wherein said BIOS instructions are executed by forcing anexternal interrupt to said microprocessor and communicating via amachine specific register.
 3. The apparatus as recited in claim 1,wherein said conditions comprise hardware detected conditions.
 4. Theapparatus as recited in claim 1, wherein said conditions furthercomprise results of executing said secure application program.
 5. Theapparatus as recited in claim 1, wherein, upon detection of said firstone or more of said conditions, said secure execution mode logicattempts to erase a data area of a secure volatile memory within saidmicroprocessor and attempts to log said detection in said securenon-volatile memory.
 6. The apparatus as recited in claim 5, wherein,upon successful erasure of said data area and logging of said detection,said secure execution mode logic transitions said microprocessor to saiddegraded mode.
 7. The apparatus as recited in claim 1, wherein saidsecure execution mode logic causes said microprocessor to transition toa shutdown mode following detection of a second one or more of saidconditions, wherein said shutdown mode can be exited only by resettingsaid microprocessor.
 8. The apparatus as recited in claim 7, wherein,upon detection of said second one or more of said conditions, saidsecure execution mode logic attempts to erase a data area of a securevolatile memory within said microprocessor, attempts to log saiddetection in said secure non-volatile memory, and forces saidmicroprocessor into said shutdown mode.
 9. A microprocessor apparatus,for executing secure code within a secure execution environment, themicroprocessor apparatus comprising: a secure non-volatile memory,configured to store a secure application program; and a microprocessor,coupled to said secure non-volatile memory via a private bus, configuredto execute non-secure application programs and said secure applicationprogram, said microprocessor comprising: secure execution mode logic,configured to monitor conditions corresponding to said microprocessorthat are associated with potential security exposure and tampering, andconfigured to cause said microprocessor to transition to a degradedoperating mode from said secure execution mode following detection of afirst one or more of said conditions, wherein said degraded operatingmode exclusively provides for execution of BIOS instructions, said BIOSinstructions comprising instructions to allow for user input and displayof messages.
 10. The microprocessor apparatus as recited in claim 9,wherein said BIOS instructions are executed by forcing an externalinterrupt to said microprocessor and communicating via a machinespecific register.
 11. The microprocessor apparatus as recited in claim9, wherein said conditions comprise hardware detected conditions. 12.The microprocessor apparatus as recited in claim 9, wherein saidconditions further comprise results of executing said secure applicationprogram.
 13. The microprocessor apparatus as recited in claim 9,wherein, upon detection of said first one or more of said conditions,said secure execution mode logic attempts to erase a data area of asecure volatile memory within said microprocessor and attempts to logsaid detection in said secure non-volatile memory.
 14. Themicroprocessor apparatus as recited in claim 13, wherein, uponsuccessful erasure of said data area and logging of said detection, saidsecure execution mode logic transitions said microprocessor to saiddegraded mode.
 15. The microprocessor apparatus as recited in claim 9,wherein said secure execution mode logic causes said microprocessor totransition to a shutdown mode following detection of a second one ormore of said conditions, wherein said shutdown mode can be exited onlyby resetting said microprocessor.
 16. The microprocessor apparatus asrecited in claim 15, wherein, upon detection of said second one or moreof said conditions, said secure execution mode logic attempts to erase adata area of a secure volatile memory within said microprocessor,attempts to log said detection in said secure non-volatile memory, andforces said microprocessor into said shutdown mode.
 17. A method forexecuting secure code within a secure execution environment, the methodcomprising: providing a secure non-volatile memory for storage of thesecure code, wherein the secure code is accessed within the securenon-volatile memory via private transactions accomplished over a privatebus that is coupled between the secure non-volatile memory and amicroprocessor, and wherein the private bus is isolated from all systembus resources within the microprocessor and external to themicroprocessor, and wherein the private bus is observable and accessibleexclusively by secure execution logic within the microprocessor;monitoring conditions corresponding to the microprocessor that areassociated with potential security exposure and tampering; and firsttransitioning the microprocessor to a degraded operating mode from thesecure execution mode following detection of a first one or more of theconditions, wherein the degraded operating mode exclusively provides forexecution of BIOS instructions, the BIOS instructions comprisinginstructions to allow for user input and display of messages.
 18. Themethod as recited in claim 17, wherein the BIOS instructions areexecuted by forcing an external interrupt to the microprocessor andcommunicating via a machine specific register.
 19. The apparatus asrecited in claim 17, wherein the conditions comprise hardware detectedconditions.
 20. The apparatus as recited in claim 17, wherein theconditions further comprise results of executing the secure applicationprogram.
 21. The apparatus as recited in claim 17, wherein said firsttransitioning comprises: upon detection of the first one or more of theconditions, first attempting to erase a data area of a secure volatilememory within the microprocessor and second attempting to log thedetection in the secure non-volatile memory.
 22. The apparatus asrecited in claim 22, wherein, upon successful erasure of the data areaand logging of the detection, completing said first transitioning. 23.The apparatus as recited in claim 17, further comprising: secondtransitioning the microprocessor to a shutdown mode following detectionof a second one or more of the conditions, wherein the shutdown mode canbe exited only by resetting the microprocessor.
 24. The apparatus asrecited in claim 23, wherein, upon detection of the second one or moreof the conditions, first attempting to erase a data area of a securevolatile memory within the microprocessor, second attempting to log thedetection in the secure non-volatile memory, and forcing themicroprocessor into the shutdown mode.